3-4 weeks is a good time. I've kept my cherries in 80*F and high 60's*F at different times. You have a trade off. The higher the temp, the faster the eggs develop and hatch, the faster they grow, but they faster they burn out and die too. With higher temps you also risk bacterial infections.
Since I selectively breed my neos, I like to speed up the generations so I can get more done in a shorter tiem period. However that means I have to constantly be aware of water conditions/shrimp health as well.